Ion channels are a class of proteins that play critical role in cellular signaling by mediating the excitability of cells. This activity of ion channels is not static, but constantly changes in response to various intracellular and extracellular signals and, in recent years, many of the factors that directly modulate the activity of ion channels have begun to be identified. Understanding the chemical basis for how the activity of these proteins is modulated represents one of the most important areas of modern science as it impacts our understanding of how the nervous system works as well as our ability to pharmacologically manipulate it. The proposed research seeks to characterize several potential modes of modulation of the human ether-a-go-go related gene (HERG) ion channel, a voltage-gated potassium channel found in brain and heart. The activity of HERG channels is modulated by oxygen (02) and reactive oxygen species (ROS) in vitro and it has been proposed that HERG channels function as direct sensors of O2/ROS in vivo. The hypothesis that O2/ROS sensing by HERG channels is mediated by the reversible oxidation of methionine residues within the channel will be evaluated by characterizing the biophysical effects of site-specifically incorporating methionine sulfoxide into the channel on channel biophysics. HERG channel activity is also modulated by various classes of small molecules in vivo. In all but a few cases, the binding site(s) for these compounds have not been identified, however. The hypothesis that small molecules modulate the activity of HERG channels by binding to the N-terminal (PAS) domain will be evaluated using NMR spectroscopy (to detect structural perturbations associated with ligand binding) and electrophysiology (to define biophysical changes associated with ligand binding). Finally, HERG channel activity also appears to be modulated by various protein-protein interactions. To determine the site(s) in HERG that interact with other proteins, photoreactive amino acids capable of forming protein cross-links will be site-specifically incorporated into the HERG.